igc: fix tunnel offloading

Checking tunnel offloading, it turns out that offloading doesn't work
as expected.  The following script allows to reproduce the issue.
Call it as `testscript DEVICE LOCALIP REMOTEIP NETMASK'

=== SNIP ===
if [ $# -ne 4 ]
then
  echo "Usage $0 DEVICE LOCALIP REMOTEIP NETMASK"
  exit 1
fi
DEVICE="$1"
LOCAL_ADDRESS="$2"
REMOTE_ADDRESS="$3"
NWMASK="$4"
echo "Driver: $(ethtool -i ${DEVICE} | awk '/^driver:/{print $2}') "
ethtool -k "${DEVICE}" | grep tx-udp
echo
echo "Set up NIC and tunnel..."
ip addr add "${LOCAL_ADDRESS}/${NWMASK}" dev "${DEVICE}"
ip link set "${DEVICE}" up
sleep 2
ip link add vxlan1 type vxlan id 42 \
		   remote "${REMOTE_ADDRESS}" \
		   local "${LOCAL_ADDRESS}" \
		   dstport 0 \
		   dev "${DEVICE}"
ip addr add fc00::1/64 dev vxlan1
ip link set vxlan1 up
sleep 2
rm -f vxlan.pcap
echo "Running tcpdump and iperf3..."
( nohup tcpdump -i any -w vxlan.pcap >/dev/null 2>&1 ) &
sleep 2
iperf3 -c fc00::2 >/dev/null
pkill tcpdump
echo
echo -n "Max. Paket Size: "
tcpdump -r vxlan.pcap -nnle 2>/dev/null \
| grep "${LOCAL_ADDRESS}.*> ${REMOTE_ADDRESS}.*OTV" \
| awk '{print $8}' | awk -F ':' '{print $1}' \
| sort -n | tail -1
echo
ip link del vxlan1
ip addr del ${LOCAL_ADDRESS}/${NWMASK} dev "${DEVICE}"
=== SNAP ===

The expected outcome is

  Max. Paket Size: 64904

This is what you see on igb, the code igc has been taken from.
However, on igc the output is

  Max. Paket Size: 1516

so the GSO aggregate packets are segmented by the kernel before calling
igc_xmit_frame.  Inside the subsequent call to igc_tso, the check for
skb_is_gso(skb) fails and the function returns prematurely.

It turns out that this occurs because the feature flags aren't set
entirely correctly in igc_probe.  In contrast to the original code
from igb_probe, igc_probe neglects to set the flags required to allow
tunnel offloading.

Setting the same flags as igb fixes the issue on igc.

Fixes: 34428dff ("igc: Add GSO partial support")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Tested-by: Corinna Vinschen <vinschen@redhat.com>
Acked-by: Sasha Neftin <sasha.neftin@intel.com>
Tested-by: Nechama Kraus <nechamax.kraus@linux.intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>